1. Field of the Invention
This invention pertains to communications systems, and particularly to communication system which employ a pilot pattern for functions such as synchronization, channel estimation, and/or device identification.
2. Related Art and Other Considerations
Orthogonal frequency division multiplexing (OFDM) is a special subset of Frequency Division Multiplex (FDM) and is a multi-carrier modulation scheme. In orthogonal frequency division multiplexing (OFDM), the data is simultaneously encoded over various sub-carriers. A data stream is split into N parallel streams of reduced data rate and each parallel stream is transmitted on a separate sub-carrier. When the subcarriers have appropriate spacing to satisfy orthogonality (e.g., the sub-carriers' frequencies differ from each other by integer multiples of the base (lowest) sub-carrier frequency), the carriers are mutually orthogonal to each other and their spectra overlap.
Thus, in an Orthogonal Frequency Division Multiplexing (OFDM) system, the data symbols are modulated onto the orthogonal time-frequency units defined by the sub-carriers of an OFDM symbol. The duration of an OFDM symbol is usually designed to be short enough so that the propagation channel remains unchanged. Within each OFDM symbol, the available bandwidth is divided into a number of orthogonal sub-carriers onto which the above-mentioned data symbols are modulated.
The coherent demodulation of these data symbols at the receiver requires the knowledge of the slowly-varying complex-valued channel gain each data symbol experiences. This knowledge is usually obtained by introducing into the overall transmit waveform a known signal from which the receiver can estimate the channel gain. The design of such known signal, or pilot signal, for the purpose of coherent demodulation is one of several important tasks in the complete synchronization scheme of an OFDM system.
In addition to channel estimation, the other two major synchronization tasks are the initial time-frequency offsets synchronization and the identification of the communicating devices within the system. The former is required in the beginning of a communication session when the timing and frequency synchronization have not been established yet. The latter is required continuously by all communicating devices to detect and identify each other.
Many of the existing synchronization signal designs in the existing OFDM systems have separate pilot signals for channel estimation and initial time-frequency synchronization. In view of the two separate sets of pilot signals, these existing systems engage in a two-step (two-phase) synchronization process. In general, as a first step and for initial time-frequency synchronization and device identification a device-specific pilot signal is transmitted periodically with a certain duty cycle. This device-specific pilot can be a pseudo random sequence as used in a Code Division Multiple Access (CDMA) system, or any other signal with enough bandwidth, length and energy. The device-specific pilot signal is sometimes referred to as a preamble since it precedes the second phase, i.e., the data transmission phase. In the data transmission phase, channel estimation pilot symbols are uniformly distributed across a time frequency plane, as shown in FIG. 9. The time-frequency plot of FIG. 9 is simply an illustration of the frequency contents as a function of time for a time domain signal. Roughly speaking, the frequency component in the mth sub-carrier centered at mfs Hz of the nth OFDM symbol transmitted at time nTs sec. can be expressed by Equation 1.λ(n,m)p(t−nTs)ej2πmfst,  Equation (1)In Equation 1, p(t) is a narrow band pulse shaping function (usually a rectangular pulse), Ts is the OFDM symbol duration, fs≈1/Ts is the sub-carrier spacing and λ(n, m) is the associated symbol value which is set to 1 for the channel estimation pilot symbol.
Typically, to achieve synchronization in the first phase, the receiver first matches the received signal to one of the possible preambles with a certain hypothesized time-frequency offset, and calculates a corresponding metric. The same calculation is performed for all possible preambles with all possible time-frequency offsets. The identity of the transmitter and its time-frequency offset with respect to the receiver are then determined by selecting those parameters associated with the largest metric. Thus the objectives of the first phase (device identification and initial synchronization achieved).
After the initial synchronization is established, in the data transmission phase the receiver then locates the channel estimation pilot symbols from which the channel's time-frequency response H (t, f) can be interpolated for the coherent demodulation of a data symbol transmitted at time t in the sub-carrier centered at frequency f. The channel estimation pilot symbols need to be placed frequently enough in the time-frequency plane to ensure the quality of the interpolation. From the sampling point of view, this implies that the pilot symbol insertion rate should be greater than the maximum delay spread in the frequency domain and the maximum Doppler spread in the time domain such that there will be no aliasing for the channel's delay-Doppler response which is defined by Equation 2.h(τ,ν)Δ∫∫H(t,f)ej2πτfe−j2πvtdtdf.  Equation (2)
A straightforward approach to meet these requirements is to use a regularly spaced pilot pattern such as that shown in FIG. 10(a). As long as 1/Tp and 1/fp satisfy the above-mentioned criteria, the channels delay-Doppler response (shown as contour curves in FIG. 10(b)) will be aliasing free and can therefore be recovered with a two-dimensional low pass filter (interpolator). Table 1 summarizes these notations with values from an example scenario. FIG. 11 gives a visual illustration of their relative scales in the time-frequency (or delay-Doppler) plane.
TABLE 1Summary of NotationsParameterCommentValue (example) τmaximal delay spread7.8125 μsec vmaximal Doppler spread500 HzTppilot insertion period in time domain2 msec(1/ v)fppilot insertion period in freq. domain128 kHz(1/ τ) v τpilot density 1/256Ts = Tp/NOFDM symbol duration 2/16 msecfs = fp/M  sub  ⁢      -    ⁢  carrier  ⁢          ⁢      spacing    ⁢                              ⁢                            (          ≈              1                  T          s                      )   128/16 kHz
The use of preambles for initial synchronization and device identification has three primary draw-backs. First of all, since these preambles are not continuously transmitted, the receiver has to keep searching until one preamble (or sometimes more if the signal is weak) is available, thereby delaying the acquisition time. Secondly, dedicating all available bandwidth over a time interval entirely to a single purpose is not an efficient way of utilizing the radio resource. The two-step (preamble followed by channel estimation pilot) solution fails to exploit the underlying OFDM structure, which allows for the precise design of the synchronization signal's time-frequency contents. Finally, the preamble may not be bandwidth scalable, which is of great importance for the future systems that may operate in a wide variety of spectrum allocation scenario.
The regularly spaced pilot, on the other hand, is more scalable since it can be easily extended in frequency and a “spectrum hole” can be created by skipping a few pilot sub-carriers. The regularly spaced pilot is thus typically used only for channel estimation or perhaps for initial synchronization if the receiver is allowed to observe it over a long period of time. Only in rare instances do the pilot signals of the data transmission phase provide any type of device identification (e.g., identification of a transmitter).
For example, in U.S. Pat. No. 6,961,364 to Laroia et al., the slope of a pilot tone hopping sequence is employed to identify a transmitting base station having the strongest downlink signal. Each base station transmits a Latin Squares pilot signal with a locally unique slope. A mobile user unit performs base station identification by estimating the slope of the strongest received pilot signal. In addition, the mobile user unit can synchronize to the pilot signal by estimating its initial frequency shift. Inherent delays resulting when using a training sequence of symbols is not experienced when using the pilot tone hopping sequence to identify the base station having the strongest downlink signal.
What is needed, therefore, and an object of the present invention, is an effective and efficient system, method, and apparatus for using a single pilot pattern for plural synchronization tasks, including device identification.